These teachings relate generally to detectors of electromagnetic radiation and, more specifically, relate to photodiode detectors that are responsive to electromagnetic radiation in more than one spectral band. Even more specifically, these teachings relate to detectors of electromagnetic radiation that have an electrically tunable response to light of different wavelengths.
Electromagnetic radiation detectors that are responsive to light in more than one wavelength band, also referred to as multi-spectral or multi-color detectors, provide a number of advantages in modern imaging systems. In general, the light that is detected may be visible light or light that is not visible to the human eye (e.g., infrared (IR) radiation).
Early efforts to detect IR radiation within more than one spectral band have relied on the use of multiple detector arrays, each having a different spectral filter. Multiple detector arrays with different spectral responses have also been used. The use of a continuously variable wedge filter in conjunction with a detector array is also known in the art, as is the use of a mechanical spectral filter selector. For reasons related at least to increased cost, complexity and weight, these conventional approaches to multi-spectral imaging are disadvantageous for many applications.
It was thus realized that the detection of IR radiation in two or more spectral bands with a single integrated detector device was a very desirable alternative to the conventional approaches. Representative examples of such detectors can be found in the following commonly assigned U.S. Patents.
In U.S. Pat. No. 5,113,076 by Eric Schulte, xe2x80x9cTwo terminal multi-band infrared radiation detectorxe2x80x9d, there is described a radiation detector that includes a first heterojunction and a second heterojunction that are electrically coupled together in series between a first electrical contact and a second electrical contact. The detector contains at least a three regions or layers, including a first layer having a first type of electrical conductivity, a second layer having a second type of electrical conductivity and a third layer having the first type of electrical conductivity. The first and second heterojunctions are coupled in series and function electrically as two back-to-back diodes. During use the detector is coupled to a switchable bias source that includes a source of positive bias (+Vb) and a source of negative bias (xe2x88x92Vb). With +Vb applied across the detector the first heterojunction is in far forward bias and functions as a low resistance conductor, thereby contributing no significant amount of photocurrent to the circuit. The second heterojunction is in a reverse bias condition and modulates the circuit current in proportion to the photon flux of an associated spectral region or color. Conversely, with xe2x88x92Vb applied across the detector the second heterojunction is in forward bias and contributes no significant photocurrent to the circuit while the first heterojunction is reversed biased and produces a current modulation proportional to the incident flux, where the flux is associated with a different spectral region.
In U.S. Pat. No. 5,731,621 to Kenneth Kosai, xe2x80x9cThree band and four band multispectral structures having two simultaneous signal outputsxe2x80x9d, there is described a solid state array that has a plurality of radiation detector unit cells, wherein each unit cell includes a bias-selectable two color photodetector in combination with either a second bias-selectable two color detector or a single photodetector. Each unit cell is thus capable of simultaneously outputting charge carriers resulting from the absorption of electromagnetic radiation within two spectral bands that are selected from one of four spectral bands or three spectral bands.
In U.S. Pat. No. 5,751,005 by Richard Wyles et al., xe2x80x9cLow-crosstalk column differencing circuit architecture for integrated two-color focal plane arraysxe2x80x9d, there is described an integrated two-color staring focal plane array having rows and columns of photodetector unit cells, each of which is capable of simultaneously integrating photocurrents resulting from the detection of two spectral bands. A readout circuit performs a subtraction function, and includes a differential charge-sensing amplifier in a one-per-column arrangement. The amplifier works in cooperation with circuitry located in each unit cell. The subtraction function is employed to create a separate Band1 signal from a Band2 and (Band1+Band2) signals generated by each simultaneous two-color detector. The circuit offers low spectral crosstalk between the two spectral bands.
Also by example, in U.S. Pat. No. 5,959,339 by Chapman et al., xe2x80x9cSimultaneous two-wavelength p-n-p-n infrared detectorxe2x80x9d there is disclosed an array that contains a plurality of radiation detectors. Each radiation detector includes a first photoresponsive diode (D1) having an anode and a cathode that is coupled to an anode of a second photoresponsive diode (D2). The first photoresponsive diode responds to electromagnetic radiation within a first band of wavelengths and the second photoresponsive diode responds to electromagnetic radiation within a second band of wavelengths. Each radiation detector further includes a first electrical contact that is conductively coupled to the anode of the first photoresponsive diode; a second electrical contact that is conductively coupled to the cathode of the first photoresponsive diode and to the anode of the second photoresponsive diode; and a third electrical contact that is conductively coupled to a cathode of each second photoresponsive diode of the array. The electrical contacts are coupled during operation to respective bias potentials. The first electrical contact conducts a first electrical current induced by electromagnetic radiation within the first predetermined band of wavelengths, and the second electrical contact conducts a second electrical current induced by electromagnetic radiation within the second predetermined band of wavelengths, less an electrical current induced by electromagnetic radiation within the first predetermined band of wavelengths.
The disclosures of these various commonly assigned U.S. Patents are incorporated by reference herein in so far as there is no conflict with the teachings of this invention.
Also of interest to the teachings of this invention is a p-i-i-n (p-type, intrinsic, intrinsic, n-type) detector that is described by Brxc3xcaggermann et al., xe2x80x9cThe operational principle of a new amorphous silicon based p-i-i-n color detectorxe2x80x9d, J. Appl. Phys. 81(11), 1 Jun. 1997, 7666-7672. The device is constructed using two large band gap front layers of doped and intrinsic hydrogenated amorphous silicon carbide (a-SiC:H), followed by an intrinsic and a doped a-Si:H layer. These authors report that by band gap engineering an experimental red response is maximized at a large reverse bias voltage, whereas the green response has its maximum at low reverse bias voltage. The potential profile of the p-i-i-n structure is said to be of crucial importance to the color detection mechanism. At larger wavelengths the large potential drop across the two highly defective front layers assists recombination in the back part of the device, which leads to the drop in the red response at low reverse voltage. For the voltage-dependent shift in spectral sensitivity it is said to be important that photogenerated carriers, under green bias illumination, are lost by recombination in the front part of the device.
Also of interest is an n-i-p-i-i-n detector of a type described by H. Stiebig et al., xe2x80x9cTransient Behavior of Optimized nipiin Three-Color Detectorsxe2x80x9d, IEEE Transactions on Electron Devices, Vol. 45, No. 7, July 1998, 1438-1444. These authors report the detection of the fundamental components of visible light (blue, green, red) with a multi-spectral two-terminal photodiode that is based on amorphous silicon. The preferential carrier collection region of the two-terminal device shifts upon a change of the applied bias voltage, which leads to a color sensitivity. Structures with controlled bandgap and mobility-lifetime product exhibit a dynamic behavior above 96 dB. Three linearly independent spectral response curves can be extracted to generate a RGB (red-green-blue)-signal. Bias voltage switching experiments under different monochromatic illumination conditions were carried out to investigate the time-dependent behavior.
The foregoing and other problems are overcome, and other advantages are realized, in accordance with the presently preferred embodiments of these teachings.
A photodetector in accordance with the teachings of this invention includes a substrate having a surface; a first layer of semiconductor material that is disposed above the surface, the first layer containing a first dopant at a first concentration for having a first type of electrical conductivity; and a second layer of semiconductor material overlying the first layer. The second layer contains a second dopant at a second concentration for having a second type of electrical conductivity and forms a first p-n junction with the first layer. The second layer is compositionally graded through at least a portion of a thickness thereof from wider bandgap semiconductor material to narrower bandgap in a direction away from the p-n junction. The compositional grading can be done in a substantially linear fashion, or in a substantially non-linear fashion, e.g., in a stepped manner. Preferably the first dopant concentration is at least an order of magnitude greater than the second concentration, and more preferably is at least two orders of magnitude greater. When the first p-n junction is reverse biased, a depletion region exists substantially only within the second layer, and varying the magnitude of the bias shifts the wavelength at which a maximum spectral sensitivity or responsiveness is obtained. At least one electrical contact is provided for coupling the second layer to a source of variable bias voltage for reverse biasing the p-n junction. As the magnitude of the bias voltage is changed a wavelength of electromagnetic radiation to which the photodetector is responsive is changed.
As examples, the semiconductor material can be selected from a Group II-VI material or from a Group III-V material. The first type of electrical conductivity can be p-type, and the second type of electrical conductivity can be n-type, or the first type of electrical conductivity can be n-type, and the second type of electrical conductivity can be p-type.
The photodetector can further include a third layer of semiconductor material that is disposed above the second layer, the third layer containing a third dopant at a third concentration for having the first type of electrical conductivity and a fourth layer of semiconductor material overlying the third layer. The fourth layer contains a fourth dopant at a fourth concentration for having the second type of electrical conductivity and forming a second p-n junction with the third layer, the fourth layer being compositionally graded through at least a portion of a thickness thereof from wider bandgap semiconductor material to narrower bandgap semiconductor material in a direction away from the second p-n junction. The third concentration is at least an order of magnitude greater than the fourth concentration, and when the second p-n junction is reverse biased a depletion region exists substantially only within the fourth layer.
Also disclosed is an array of IR radiation responsive photodetectors wherein each photodetector includes a photodiode having a p-n junction. A wavelength at which a maximum spectral response of the photodiode occurs is determined at least in part by a magnitude of a reverse bias voltage applied to the p-n junction. Each of the photodiodes includes a layer of semiconductor material that is compositionally graded from wider bandgap material towards narrower bandgap material in a direction away from the p-n junction. The compositionally graded layer confines substantially all of a depletion region of the photodiode.
A method is also disclosed for operating an array of electromagnetic radiation responsive photodetectors. The method includes providing the array such that each photodetector includes a photodiode having a p-n junction, where a wavelength at which a maximum spectral response of the photodiode occurs is determined at least in part by a magnitude of a reverse bias voltage applied across the p-n junction. Each of the photodiodes includes a layer of semiconductor material that is compositionally graded from wider bandgap material towards narrower bandgap material in a direction away from the p-n junction, where the layer confines substantially all (e.g., preferably more than about 95%, and more preferably more than about 99%) of a depletion region of the photodiode. During operation of the array the method establishes for each photodetector a predetermined magnitude of reverse bias voltage; and detects a signal generated from each photodetector that results from incident electromagnetic radiation having wavelengths that correspond to the maximum spectral response that is determined at least in part by the magnitude of the reverse bias voltage. The step of establishing may establish approximately the same magnitude of reverse bias voltage for each photodetector of the array, or it may establish approximately the same magnitude of reverse bias voltage for some of the photodetectors of the array while establishing at least one different magnitude of reverse bias voltage for other photodetectors of the array, or the step of establishing may establish a different magnitude of reverse bias voltage for each photodetector of the array. For a case where the array contains rows and columns of photodetectors, the step of establishing may establish a different magnitude of reverse bias voltage for individual ones of rows or columns of the array. The step of establishing can include varying the magnitude of the reverse bias potential during operation of the array. For a case where the layer of semiconductor material is compositionally graded in a stepped fashion, increments of reverse bias voltage can have a magnitude that is related to the steps.
An alternating current signal can be superimposed on the reverse DC bias voltage and a synchronous detection technique used to detect photons corresponding to a certain bandgap energy.